Method for forming multi-layer coating film

ABSTRACT

This invention relates to a process for forming a multi-layer coating film by applying, to a substrate, intermediate paint, metallic paint and clear paint by 3C1B method, which process is characterized in that said intermediate paint contains 0.5 to 5 phr of flat talc powder of a size of 0.5 to 10 μm in longer direction and 0.01 to 1 μm in thickness, and has a total pigment content of 40 to 100 phr.

TECHNICAL FIELD

[0001] This invention relates to a process for forming a multi-layercoating film which comprises an intermediate coating film, a metalliccoating film and a clear coating film, and which has chipping-resistanceand smoothness improved.

BACKGROUND ART

[0002] It has already been known to form, on exterior surfaces ofautomobile body etc., a multi-layer coating film by 3C1B method, i.e.,by applying intermediate paint, metallic paint and clear paint in thisorder, and curing the resultant coatings simultaneously by heating. Itis often seen that this multi-layer coating film is hit by gravelsspattered by a running car, and thus partially peels off (whichphenomenon is called “chipping”). In order to resolve this chipping, ithas been proposed to place flexible coating film between coating filmlayers. This method is, however, undesirable since it increases thenumber of coating process steps.

[0003] The objective of this invention is to improve, without increasingthe number of coating process steps, the chipping-resistance of amulti-layer coating film which is formed by 3C1B method with use ofintermediate paint, metallic paint and clear paint.

DISCLOSURE OF INVENTION

[0004] As a result of assiduous study, the inventors of this inventionhave now found out that the above-mentioned objective is achieved whenthe intermediate paint contains 0.5 to 5 phr of flat talc powder of aspecific size and has a total pigment content of 40 to 100 phr, and,thus, the inventors have completed this invention.

[0005] This invention relates to a process for forming a multi-layercoating film by applying, to a substrate, intermediate paint, metallicpaint and clear paint by 3C1B method, which process is characterized inthat said intermediate paint contains 0.5 to 5 phr of flat talc powderof a size of 0.5 to 10 μm in longer direction and 0.01 to 1 μm inthickness, and has a total pigment content of 40 to 100 phr.

[0006] In the following, the process of this invention for forming amulti-layer coating film is explained in more detail.

MODES FOR CARRYING OUT INVENTION

[0007] The characteristic feature of this invention resides in the useof intermediate paint which contains 0.5 to 5 phr of flat talc powder ofa size of 0.5 to 10 μm in longer direction and 0.01 to 1 μm inthickness, and which has a total pigment content of 40 to 100 phr. Owingto this characteristic feature, a multi-layer coating film hassuccessfully been improved in both chipping-resistance and smoothnesswithout increase in the number of coating process steps.

[0008] Concrete examples of the intermediate paint which is used in thisinvention may include organic solvent type or aqueous type liquid paintwhich is obtained by mixing, with organic solvent and/or water, a baseresin such as polyester resin, alkyd resin and acrylic resin having acrosslinking functional group such as hydroxyl group; a crosslinkingagent such as melamine resin and blocked polyisocyanate compound; andflat talc of a specific size; and other pigments. Said other pigmentsinclude coloring pigments and extender pigments other than flat talc.The size (particle diameter) of these other pigments is suitablyequivalent to, or less than, the size of the above-mentioned flat talcpowder.

[0009] In this specification, “phr” is an abbreviation of “part perhundred parts of resin”, which means part(s) by weight per 100 parts byweight of resin solid content of paint.

[0010] The flat talc powder which is used in this invention is aninorganic extender pigment having hydrous magnesium silicate as a maincomponent. Said powder are flakes of a size of 0.5 to 10 μm, preferably1 to 5 μm, in longer direction, and 0.01 to 1 μm, preferably 0.1 to 0.5μm, in thickness. When the size in longer direction of the flat talcpowder used is smaller than 0.5 μm, the chipping resistance of coatingfilm decreases, while, when it is larger than 10 μm, the smoothness ofcoating film decreases, both of which are undesirable. When talc isreplaced by a powder of clay, barium sulfate or mica, it becomesimpossible to achieve the object of this invention to simultaneouslyimprove the chipping resistance and smoothness of a multi-layer coatingfilm.

[0011] The content of flat talc powder in intermediate paint may be in arange of 0.5 to 5 parts by weight per 100 parts by weight of resin solidcontent of paint (i.e., 0.5 to 5 phr), preferably 1 to 4 parts by weight(1 to 4 phr). When said content is lower than 0.5 part by weight, thechipping resistance of coating film is not improved, while, when it ishigher than 5 parts by weight, the smoothness of coating film decreases,both of which are undesirable.

[0012] As other pigments to be compounded in intermediate paint togetherwith flat talc powder, any known coloring pigments and extender pigmentswhich are usually used in intermediate paint are usable. As for thecontent of said other pigments, the total content of flat talc powderand such other pigments, i.e., “total pigment content”, may be 40 to 100phr, preferably 60 to 97 phr, especially desirably 80 to 95 phr. Whensaid total pigment content in intermediate paint used is lower than 40phr, the chipping resistance of coating film decreases, while, when itis larger than 100 phr, the multi-layer film becomes mechanicallybrittle, both of which are undesirable.

[0013] This intermediate paint may be applied on a metal-made orplastic-made substrate for automobile body on which an under coatingsuch as cationic electrodeposition paint has been applied ascircumstances may demand. Paint application may be conducted by anyknown method. The thickness of coating film of this intermediate paintis suitably 15 to 40 μm, in particular 20 to 35 μm, as a cured film.

[0014] Subsequently, while intermediate coating film is kept uncured,this uncured intermediate coating surface is coated with metallic paint.

[0015] Any known metallic paint is usable, examples of which includeusual thermosetting metallic paint which is obtained by mixing, withorganic solvent and/or water, a base resin such as polyester resin,alkyd resin and acrylic resin having a crosslinking functional groupsuch as hydroxyl group; a crosslinking agent such as melamine resin andblocked polyisocyanate compound, a metallic pigment such as aluminumflake and titanium oxide-coated mica; and, furthermore, a coloringpigment and an extender pigment as circumstances may demand. Metallicpaint is applied by a usual method on the above-mentioned uncuredcoating surface of intermediate paint. The thickness of coating film ofthis metallic paint is suitably 10 to 40 μm, preferably 15 to 35 μm, asa cured film.

[0016] Subsequently, while the coating film of this metallic paint iskept uncured, clear paint is further applied.

[0017] Any known clear paint which forms colorless or coloredtransparent coating film is usable, examples of which include usualthermosetting clear paint which is obtained by mixing, with organicsolvent, a base resin such as polyester resin, alkyd resin and acrylicresin having a crosslinking functional group such as hydroxyl group; acrosslinking agent such as melamine resin and blocked polyisocyanatecompound; and, furthermore, as circumstances may demand, a coloringpigment or an extender pigment which does not substantially interferewith the transparency of coating film. Clear paint is applied by a usualmethod on the above-mentioned uncured coating surface of metallic paint.The thickness of coating film of this clear paint is generally 20 to 80μm, preferably 25 to 50 μm, as a cured film.

[0018] When intermediate paint, metallic paint and clear paint areapplied in order in this manner to form an uncured three-layer coatingfilm, and when the three layers of coating film are simultaneously curedby heating at a temperature of about 100 to about 180° C., in particularat about 120 to about 160°C., for about 10 to 40 minutes, a multi-layercoating film as an object of this invention is formed.

[0019] According to the afore-mentioned process of this invention, thereis formed, without increase in the number of process steps, amulti-layer coating film which is excellent in both chipping resistanceand smoothness, by 3C1B method with use of intermediate paint, metallicpaint and clear paint, said intermediate paint containing 0.5 to 5 phrof flat talc powder of a size of 0.5 to 10 μm in longer direction and0.01 to 1 μm in thickness, and having a total pigment content of 40 tol00 phr.

EXAMPLE

[0020] In the following, this invention is explained in more detail bymeans of Examples and Comparative Examples. Both part and % are onweight basis, and the thickness of coating film is that of cured coatingfilm.

Example 1

[0021] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin melamine resin-type intermediate paint [containing, in an organicsolvent, 3 phr of flat talc of a size of 5 μm in longer direction and0.5 μm in thickness, 80 phr of titanium white (particle size: 0.2 μm)and 1 phr of carbon black (particle size: 0.1 μm)] to a film thicknessof 30 μm. After left still for two minutes at a room temperature, thuscoated steel plate was further coated with thermosetting acrylic resintype metallic paint (film thickness: 15 μm) and thermosetting acrylicresin type clear paint (film thickness: 40 μm) in order, and,thereafter, thus applied three layers of coating film were heated at140° C. for 30 minutes and were thereby simultaneously cured. Thusobtained multi-layer coating film was excellent in both chippingresistance and smoothness (sense of gloss).

Comparative Example 1

[0022] A multi-layer coating film was formed by the same operation as inExample 1 except that flat talc was not compounded in intermediatepaint. Thus obtained multi-layer film was equivalent to that of Example1 in smoothness, but was inferior thereto in chipping resistance.

Comparative Example 2

[0023] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin melamine resin-type intermediate paint [containing, in an organicsolvent, 3 phr of clay (particle size: 0.1 to 2 μm), 80 phr of titaniumwhite (particle size: 0.2 μm) and 1 phr of carbon black (particle size:0.1 μm)] to a film thickness of 30 μm. After left still for two minutesat a room temperature, thus coated steel plate was further coated withthermosetting acrylic resin type metallic paint (film thickness: 15 μm)and thermosetting acrylic resin type clear paint (film thickness: 40 μm)in order, and, thereafter, thus applied three layers of coating filmwere heated at 140° C. for 30 minutes and were thereby simultaneouslycured. Thus obtained multi-layer coating film was equivalent to that ofExample 1 in smoothness, but was inferior thereto in chippingresistance.

Comparative Example 3

[0024] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin melamine resin-type intermediate paint [containing, in an organicsolvent, 3 phr of barium sulfate (particle size: 0.1 to 2 μm), 80 phr oftitanium white (particle size: 0.2 μm) and 1 phr of carbon black(particle size: 0.1 μm)] to a film thickness of 30 μm. After left stillfor two minutes at a room temperature, thus coated steel plate wasfurther coated with thermosetting acrylic resin type metallic paint(film thickness: 15 μm) and thermosetting acrylic resin type clear paint(film thickness: 40 μm) in order, and, thereafter, thus applied threelayers of coating film were heated at 140° C. for 30 minutes and werethereby simultaneously cured. Thus obtained multi-layer coating film wasequivalent to that of Example 1 in smoothness, but was inferior theretoin chipping resistance.

Comparative Example 4

[0025] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin melamine resin-type intermediate paint [containing, in an organicsolvent, 3 phr of flat mica (5 μm in longer direction, and 0.5 μm inthickness), 80 phr of titanium white (particle size: 0.2 μm) and 1 phrof carbon black (particle size: 0.1 μm)] to a film thickness of 30 μm.After left still for two minutes at a room temperature, thus coatedsteel plate was further coated with thermosetting acrylic resin typemetallic paint (film thickness: 15 μm) and thermosetting acrylic resintype clear paint (film thickness: 40 μm) in order, and, thereafter, thusapplied three layers of coating film were heated at 140° C. for 30minutes and were thereby simultaneously cured. Thus obtained multi-layercoating film was inferior to that of Example 1 in both smoothness andchipping resistance.

Comparative Example 5

[0026] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin - melamine resin-type intermediate paint [containing, in anorganic solvent, 3 phr of flat talc of a size of 15 μm in longerdirection and 0.5 μm in thickness, 80 phr of titanium white (particlesize: 0.2 μm) and 1 phr of carbon black (particle size: 0.1 μm)] to afilm thickness of 30 μm. After left still for two minutes at a roomtemperature, thus coated steel plate was further coated withthermosetting acrylic resin type metallic paint (film thickness: 15 μm)and thermosetting acrylic resin type clear paint (film thickness: 40 μm)in order, and, thereafter, thus applied three layers of coating filmwere heated at 140° C. for 30 minutes and were thereby simultaneouslycured. Thus obtained multi-layer coating film was almost equivalent tothat of Example 1 in chipping resistance, but was inferior thereto insmoothness.

Comparative Example 6

[0027] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin·melamine resin-type intermediate paint [containing, in an organicsolvent, 3 phr of flat talc of a size of 5 μm in longer direction and0.5 μm in thickness, 30 phr of titanium white (particle size: 0.2 μm)and 1 phr of carbon black (particle size: 0.1 μm)] to a film thicknessof 30 μm. After left still for two minutes at a room temperature, thuscoated steel plate was further coated with thermosetting acrylic resintype metallic paint (film thickness: 15 μm) and thermosetting acrylicresin type clear paint (film thickness: 40 μm) in order, and,thereafter, thus applied three layers of coating film were heated at140° C. for 30 minutes and were thereby simultaneously cured. Thusobtained multi-layer coating film was inferior to that of Example 1 inchipping resistance.

Comparative Example 7

[0028] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin·melamine resin-type intermediate paint [containing, in an organicsolvent, 3 phr of flat talc of a size of 5 μm in longer direction and0.5 μm in thickness, 106 phr of titanium white (particle size: 0.2 μm)and 1 phr of carbon black (particle size: 0.1 μm)] to a film thicknessof 30 μm. After left still for two minutes at a room temperature, thuscoated steel plate was further coated with thermosetting acrylic resintype metallic paint (film thickness: 15 μm) and thermosetting acrylicresin type clear paint (film thickness: 40 μm) in order, and,thereafter, thus applied three layers of coating film were heated at140° C. for 30 minutes and were thereby simultaneously cured. Thusobtained multi-layer coating film was inferior to that of Example 1 insmoothness and chipping resistance.

Comparative Example 8

[0029] A steel plate which had been coated with cationicelectrodeposition paint and then heat-cured was coated with polyesterresin melamine resin-type intermediate paint [containing, in an organicsolvent, 20 phr of flat talc of a size of 5 μm in longer direction and0.5 μm in thickness, 75 phr of titanium white (particle size: 0.2 μm)and 1 phr of carbon black (particle size: 0.1 μm)] to a film thicknessof 30 μm. After left still for two minutes at a room temperature, thuscoated steel plate was further coated with thermosetting acrylic resintype metallic paint (film thickness: 15 μm) and thermosetting acrylicresin type clear paint (film thickness: 40 μm) in order, and,thereafter, thus applied three layers of coating film were heated at140° C. for 30 minutes and were thereby simultaneously cured. Thusobtained multi-layer coating film was almost equivalent to that ofExample 1 in chipping resistance, but was inferior thereto insmoothness.

Coating Film Property Test

[0030] The multi-layer coating films which had been formed in theabove-mentioned Example 1 and Comparative Examples 1 to 8 were measuredfor chipping resistance and coating surface smoothness by the followingmethod. Results are shown in Table below.

[0031] Chipping Resistance

[0032] There was sprayed about 500 ml of marble having a diameter of 15to 20 mm on the surface of multi-layer coating film at an incident angleof 45 degrees, at an air-spray pressure of about 4 kg/cm² and an ambienttemperature of −20° C., by using Q-G-R Gravelometer (trademark of aproduct manufactured by Q Panel Co.), and, then, the coating surface wasvisually observed. ◯ means that, whereas intermediate coating film hadslightly peeled off, electrodeposition coating film had hardly peeledoff. Δ means that, intermediate coating film had much peeled oft andthat electrodeposition coating film had partially peeled off. × meansthat almost all intermediate coating film had peeled off, and thatelectrodeposition coating film had much peeled off.

[0033] Coating Surface Smoothness

[0034] With use of a specular glossmeter, reflectance was measured whenboth incident angle and received angle were 20 degrees, and, thus, thedegree of smoothness was examined. The higher the reflectance is, thebetter is smoothness. Example Comparative Example 1 1 2 3 4 5 6 7 8Chipping ◯ X X X Δ ◯ Δ Δ ◯ resistance Coating 85 90 85 85 60 30 90 75 10surface smoothness

1. A process for forming a multi-layer coating film by applying, to asubstrate, intermediate paint, metallic paint and clear paint by 3C1Bmethod, which process is characterized in that said intermediate paintcontains 0.5 to 5 phr of flat talc powder of a size of 0.5 to 10 μm inlonger direction and 0.01 to 1 μm in thickness, and has a total pigmentcontent of 40 to 100 phr.
 2. A process of claim 1 wherein the flat talcpowder is of a size of 1 to 5 μm in longer direction.
 3. A process ofclaim 1 wherein the flat talc powder is of a size of 0.1 to 0.5 μm inthickness.
 4. A process of claim 1 wherein the flat talc powder iscontained in an amount of 1 to 4 phr.
 5. A process of claim 1 whereintotal pigment content is 80 to 95 phr.
 6. A process of claim 1 whereinthe substrate is a metal-made or plastic-made substrate for automobilebody which has been coated with cationic electrodeposition paint as anunder coating.
 7. Articles which have been coated by the process ofclaim 1.